The present invention relates to an ultrasonic diagnostic apparatus, and particularly to an ultrasonic diagnostic apparatus that displays an elastic image indicative of the hardness or softness of a biological tissue.
An ultrasonic diagnostic apparatus, which combines a normal B-mode image and an elastic image indicative of the hardness or softness of a biological tissue together and displays the result of combination, has been disclosed in, for example, Japanese Unexamined Patent Publication No. 2005-118152 or the like. In this type of ultrasonic diagnostic apparatus, the elastic image is generated in the following manner. First, ultrasound is transmitted and received to and from the biological tissue while repeating pressure from a body surface by an ultrasonic probe and its relaxation to thereby acquire echo signals. Then, a physical quantity related to the elasticity of the biological tissue is calculated based on the acquired echo signals. The physical quantity is converted into hue information to thereby form a color elastic image. Incidentally, for example, a displacement based on deformation of the biological tissue (hereinafter called simply “displacement”) or the like is calculated as the physical quantity related to the elasticity of the biological tissue.
One example of a method for calculating the physical quantity will be explained a little more. Correlation windows each having a width corresponding to a predetermined number of data are first respectively set to two echo signals on the same sound rays, different in time from each other. A correlation arithmetic operation is performed between the correlation windows to calculate the physical quantity. In Japanese Unexamined Patent Publication No. 2008-126079, for example, a correlation arithmetic operation is performed between correlation windows to thereby calculate a shift in waveform between both echo signals. This shift in waveform is assumed to be a displacement.
Incidentally, when deformation of the biological tissue is insufficient as in the case of, for example, lack of the degrees of pressure and its relaxation, or the like, the value calculated by the correlation arithmetic operation may not appear as a difference corresponding to the discrepancy in elasticity of the biological tissue. In this case, the elastic image is not brought to one on which the elasticity of the biological tissue has been reflected accurately.
On the other hand, when the degrees of the pressure and its relaxation are in excess, a lateral shift might occur in the biological tissue. Each echo signal acquired in such a case contains noise due to the lateral shift, so that there is a fear that a correlation coefficient at a correlation arithmetic operation becomes low. There is a fear that when the degrees of the pressure and its relaxation are excessive, deformation of the biological tissue is so excessive that the correlation windows set to the two echo signals are not matched with each other, thus reducing the correlation coefficient. When the correlation coefficient at the correlation arithmetic operation becomes low here, the resulting calculated value cannot be obtained as a calculated value on which the elasticity of the biological tissue has been reflected accurately. Thus, as the correlation coefficient at the correlation arithmetic operation becomes lower, the elastic image on which the elasticity of the biological tissue has been reflected accurately cannot be obtained.
The intensity of each echo signal becomes insufficient in a region small in the number of ultrasound reflectors and at a deep portion of a biological tissue, to which transmit ultrasound is hard to achieve due to its attenuation. Thus, a correlation coefficient at a correlation arithmetic operation on each echo signal insufficient in signal intensity becomes low. When the direction of the pressure by the ultrasonic probe and its relaxation does not coincide with an ultrasonic sound ray direction, the above-described lateral shift occurs. Therefore, a correlation coefficient at a correlation arithmetic operation on each echo signal acquired in such a state also becomes low. Accordingly, an elastic image on which the elasticity of the biological tissue has been reflected accurately cannot be obtained even in these cases.
Even though, however, the elastic image on which the elasticity of the biological tissue has been reflected accurately in this way are not obtained, it is difficult for an operator who has viewed the elastic image to determine whether it is of the elastic image on which the elasticity of the biological tissue has been reflected accurately.
There has therefore been disclosed in U.S. Pat. No. 6,558,324, a method wherein in order to allow an operator to easily determine whether an elastic image is of an elastic image on which the elasticity of a biological tissue has been reflected accurately, frame-by-frame averages of values each indicative of elasticity obtained by a correlation arithmetic operation are calculated and results of evaluations of “H” (high), “M” (medium) and “L” (low) are displayed with respect to their average values.
Since, however, U.S. Pat. No. 6,558,324 aims to detect that the elastic image has been formed in a state in which deformation of the biological tissue is insufficient, a worse evaluation (“L” if described concretely) is made as the average value becomes lower. On the other hand, if the average value is high, then the evaluation of “H” is made. Even when, however, the pressure to the biological tissue and its relaxation are performed excessively and the calculated value obtained by the correlation arithmetic operation low in correlation coefficient is included as described above, the elastic image on which the elasticity of the biological tissue has been reflected accurately, is not reached. Therefore, the method of U.S. Pat. No. 6,558,324 may become insufficient to make a decision as to whether the elastic image on which the elasticity of the biological tissue has been reflected accurately is reached and may lack the correctness of its decision.
It is desirable that the problems described previously are solved.